Hello,
Of course, ID always means an identificator, under which is an object stored in the database:
NodeID -> identificator of a node in your content
nodeID = ValidationHelper.GetInteger(QueryHelper.GetString("nodeid", ""), 0); -> GetInteger function returns NodeID, if the nodeid variable contains an integer number and if it doesn't, then it returns the second parameter, which is 0 in this case
SkuID -> identificator of a product
anyFileID -> identificator of some file
Best regards,
Jan Hermann